The field of the invention is the transmission of analog and digital information by electrical signals. More particularly, the invention is related to comb spectral generators which may be utilized for a wide variety of applications, including frequency-hopping communications, data links, vehicle collision avoidance systems, analog and digital encoder transmissions, selective identification systems and precise waveform generators.
A comb spectral generator is an electronic device which generates a harmonic frequency signal series for a line spectrum of frequencies that may extend well into the upper GHz ranges. Comb spectral generation may be achieved by use of a snap-back or step recovery diode (SRD) which is biased to a predetermined operating point and has an input coupled to a periodic electrical signal generator, such as a sine wave generator. The SRD is a microwave diode which has steep doping profiles and narrow junctions to maximize charge-storage effects. This leads to fast recovery of injected charge, typically in a transition period of a few tenths of a nanosecond. This results in the exceptionally efficient production of harmonics of the frequency of the input signal generator. Present day versions of the SRD have a relatively low breakdown voltage which limits the power capability of the diode, thus making communication systems based on the SRD ideal for limited range applications.
The use of an SRD for harmonic generation is a known technique, as shown in U.S. Pat. No. 3,777,271, issued Dec. 4, 1973, in the name of Frederick John Telewski, and entitled "Generation of Microwave Frequency Combs with Narrow Line Spacing." In the Telewski patent prior art is discussed in which a microwave comb generator is constructed by utilizing an SRD and associated circuit elements which are driven by a sine wave input generator.
In operation, SRD's are alternately forward and then reversed biased. When they are forward-biased current flows through the diode junction. When they are reversed-biased current is swept from the diode junction by minority charge carriers. This action produces a transient pulse which is very high in harmonics. An SRD produces frequency components which are integral multiples of the input signal frequency such that when these frequency components are plotted in an amplitude versus frequency plot, sharply defined lines representing these frequency components are formed.
As noted in the Telewski patent, SRD multipliers operate well above frequencies above 10 MHz, although signal strength in general decreases with increasing harmonic frequency order and becomes too low to be useful at about 150-200 lines. It is also noted that the minimum obtainable comb line spacing is about one-half to one percent of the frequency of the uppermost useful comb line.
The Telewski patent describes a device in which the conventional comb SRD generator is driven simultaneously by two or more signals of different frequencies to produce two or more different combs with line spacings that correspond to the respective drive signal frequencies. Since the comb generator is inherently a non-linear device, it produces the intermodulation products of the different combs which results in a composite comb that consists of lines that are spaced by the difference between the frequencies of the two driving signals.
Step-recovery diodes, or impulse train generators, are available from Hewlett Packard which may be driven at various drive frequencies of 100, 250,500 and 1000 MHz. These impulse train generators generate useful power at harmonics through 18 GHz. Some types of SRD's require an external bias, but the Hewlett Packard models designated by them as 33002A/B, 33003A/B, 33004A/B and 33005C/D are self-biased. Either an externally biased SRD or a self-biased SRD may be employed in the present invention.
U.S. Pat. No. 3,806,811 entitled "Multiple Carrier Phase Modulated Signal Generating Apparatus," issued Apr. 23, 1974 to Wallace T. Thompson employs an SRD in which a "pump" signal is phase-modulated and is used to drive the SRD so that all of the spectral lines of the comb have the same modulation on them. The addition of phase-modulation of the SRD is a feature which may be incorporated into the present invention for various applications, if desired.
High speed switches are also required in the present invention. One type of suitable switch is the "positive-intrinsic-negative diode" or PIN diode. These devices have a region of intrinsic semiconductor material (equal hole and election charge carriers) intermediate P-type and N-type regions. When a forward bias is applied across the intrinsic region, the diode resistance drops very fast allowing PIN diodes to be used as high speed switches. Through the application of appropriate voltages, these diodes may be made to conduct or inhibit the conduction of r.f. signals. PIN switches have a number of desirable features, including that they are broadband devices that are fast switching and have high isolation and ultra-low power consumption. Consequently, they have been used in a number of applications where advantage may be taken of these characteristics.
Other types of high speed switches that may be employed in the invention are Field Effect Transistor (FET) switches, particularly Gallium Arsenide switches.
PIN diodes are also useful in the present invention to provide, if desired, amplitude-limiting and phase-modulation of the comb lines.
U.S. Pat. No. 4,623-856, entitled "Incrementally Tuned R.F. Filter Having Pin Diode Switched Lines," issued Nov. 18, 1986 to Robert H Bickley, et. al., describes an r.f. filter in which the filter frequency is adjusted using PIN diodes.
U.S. Pat. No. 4,723,306, issued Feb. 2, 1988, entitled "Wide Band Transmitter for Short Electro-Magnetic Waves," issued in the name of Helmut Fuenfgelder, et al., describes a transmitter in which PIN diodes are used as electronic switches.
U.S. Pat. No. 4,342,008 entitled "Switched Tunable Frequency Multiplier," issued Jul. 27, 1982 in the name of Robert E. Hewitt, describes the use of a PIN diode for connecting the output coupling loop to a YIG tuned frequency multiplier-to-ground.
U.S. Pat. No. 5,115,215 entitled "PIN Diode Activation Method and Apparatus," issued May 19, 1992 to Floyd A. Koontz, shows a device in which PIN diodes can be selectively made conductive to shunt the input terminals of a device so that the device is thereby effectively bypassed.
PIN diode fast-switching switches are commercially available. They may be provided also by over-driving variable gain, wideband amplifiers that utilize PIN diodes. Representative amplifiers of this type are sold by Hewlett Packard under the designations HAMP 4001/4902.